Method of anodically coating aluminum

ABSTRACT

DECRIBES A METHOD OF COATING OBJECTS MADE ENTRIELY OR PARTLY OF ALUMINUM OR AN ALUMINUM ALLOY, IN WHCIH THE PART TO BE COATED IS ANODICALLY OXIDIZED IN AN ELECTROLYTE BATH CONTAINING AT LEAST ONE ALKALINE SILICATE SOLUBLE IN THE ELECTROLYTE BATH AND AT LEAST ONE ORGANIC COMPLEX-FORMING SUBSTANCE, BY THE APPLICATION OF DIRECT CURRENT, ALTERNATING CURRENT OR PULSATING CURRENT.

United States Patent U.S. Cl. 204--58 30 Claims ABSTRACT OF THE DISCLOSURE Describes a method of coating objects made entirely or partly of aluminum or an aluminum alloy, in which the part to be coated is anodically oxidized in an electrolyte bath containing at least one alkaline silicate soluble in the electrolyte bath and at least one organic complex-forming substance, by the application of direct current, alternating current or pulsating current.

The present invention relates to a method of coating parts or objects made entirely or partly of aluminium or an aluminium alloy, and to parts or objects coated by this method.

According to the present invention there is provided a method of coating a part or object made entirely or partly of' aluminium or an aluminium alloy, wherein the part that is to be coated is submitted to a process of anodic oxidation in an electrolyte bath containing at least one alkaline silicate soluble in the liquid electrolyte and at least one organic complex-forming substance, with the application of a D.C., and A.C. or a pulsed voltage.

The parts or objects that are to be coated may be of any shape. For instance, they may be in the form of sheet, mouldings, castings or foil of which the thickness may be as little as a few microns. The parts or objects that are to be coated may, as already stated, consist in their entirety or only partly of aluminium or an aluminium alloy. Hence, sheet, mouldings or foil of which only the surfaces consist of an aluminium alloy may be treated by the present method. One example that may here be mentioned is that a plastics film bearing a vapour-deposited surface film, of aluminium which is only 0.01 micron thick can be satisfactorily coated by the present method. Reference to these two extremes, viz. coating parts which are solid aluminium or aluminium alloy and parts which merely have an aluminium or aluminium alloy surface, illustrates the range of application of the present method.

If the parts or objects that are to be coated consist only partly of aluminium or an aluminium alloy, then the nature of the portion not consisting of aluminium or an aluminium alloy makes no diiference if this portion is non-conducting. The only requirement is that the nonconducting portion should bear a coherent film of aluminium or aluminium alloy. It is obvious that the nonconducting portion must not be soluble in the electrolyte. If the portion not consisting of aluminium or aluminium alloy is an electrically conducting substance, then it must be completely enveloped on every side by a coherent conducting layer of aluminium or aluminium alloy, but such a layer may contain pores of a diameter smaller than or equal to the diameter of the gas bubbles which are evolved during the anodic oxidation treatment of the part or object.

1 If the coating isto be formed on an aluminium alloy, then the composition of the alloy may vary within wide limits. For instance, the present coatings can still be deposited on alloys containing not more than 5% of alu- 3,824,159 Patented July 16, 1974 minium and these coatings will still have properties equal to those obtainable on pure aluminium.

In the present method, the liquid electrolyte is preferably water or a mixture of water and a solvent aid, such as methanol or isopropanol, but preferably dimethyl formamide, and the alkaline silicate that is soluble in the liquid electrolyte is preferably a water-soluble silicate, more preferably an alkali metal silicate, such as sodium, potassium or lithium silicate. The electrolyte bath may contain one or more such silicates. The silicate concentration in the electrolyte may widely vary, but it is preferably from 0.1 to 15%, more preferably from 5 to 9% (by Weight).

According to observations we have made the complexforming substances used in the present method appear largely to suppress the formation of aluminium hydroxide or of aluminium silicate, but they also affect the properties of the deposits in other respects which will be referred to later.

The complex-forming substances that are used in the present method must be soluble in the electrolyte employed. Should the desired complex-forming substances be insoluble or not sufficiently soluble for instance in the preferred liquid electrolyte, viz. water, then a mixture of water and one or more solvent aids may be provided, the choice of the solvent aid(s) naturally depending upon the nature of the complex-forming substance. Hence, in the presence of a solvent aid, compounds that are of limited solubility in water, such as pyridines or pyridine bases, can be readily employed.

The most important of appropriate complex-forming substances can be broadly divided into 6 groups, namely:

( 1) Water-soluble primary, secondary and tertiary amines, such as alkanolamines, mono-, diand triethanolamine, 2-aminopropanol, 3-dimethyl-2-aminoethanol, salts of ethylene diamino-tetraacetic acid, salts of cyclohexanediamino 1,2 tetraacetic acid, salts of nitrilo-triacetic acid, pyridine 2,6-dicarboxylic acid, 2- pyridyl-hydrazine, pyridine-2,6-dicarboxylic acid, 2- pyridyl-hydrazine, pyridine-S-sulphonic acid, pyrrolidone, pyrrole-Z-carboxylic acid and pyrimidine.

(2) Amino acids in the form of water-soluble salts, possibly substituted, such as glycine, alanine, glutamic acid, tryptophan, methionine, tyrosine, 3-bromotyrosine, aspartic acid, oxylysine and oxyproline.

(3) Water-soluble salts of organic carboxylic acids which may be substituted and possibly contain multiple bonds, such as maleic acid], fumaric acid, acrylic acid, methacrylic acid, cinnamic acid, pyromellitic acid, citric acid and tartaric acid.

(4) Water-soluble salts of sulphonic acids which may be substituted, such as toluenesulphonic acid, benzenesulphonic acid and lignosulphonic acid.

(5) Water-soluble monohydric or polyhydric substituted or unsubstituted phenols, such as phenol, cresol, resorcinol, 2,4,6-trinitroresorcinol, phloroglucinol andpyrogallol.

(6) Water-soluble polyhydric alcohols, possibly substituted, such as ethylene glycol, propylene glycol, poly-. propylene glycol, nitropropanediol.

Monoethanolamine has proved to be an outstandingly good complex-former. The complex-forming substances may be introduced individually or as a mixture of two or; more thereof. The concentration of the complex-forming substance may vary Within Wide limits, but will usually be from 0.1% to 40%, preferably from 1% to 12%, of the electrolytic bath.

The coatings may be formed in the present method by using a D.C. voltage, the workpieces being connected to the positive pole of a D.C. source and the bath toth e, negative pole. The present method. is therefore an anodic" oxidation of aluminium in an alkaline medium, the formation of aluminium hydroxide being suppressed by using an alkaline silicate as an electrolyte and by introducing complex-forming substances.

Alternatively the formation of the coatings by the present method may be effected by the application of a pulsed current. The use of current pulsing is preferred in cases in which electronically controlled rectifier equipment is available. According to the type of layer desired, coating is effected by immersing the part in the bath and by then controllably raising the voltage, and possibly thereby keeping the current density constant, or by slowly lowering the part into the bath whilst the voltage remains constant.

An alternating current can also be used, particularly for coating foil or thin surfaces, and the materials thus obtained can be very suitably used in the making of capacitors.

If a polyphase AC. is used, say a three-phase A.C., it will be readily understood that all three electrodes can be coated simultaneously.

Furthermore, it is naturally also possible to make use of a direct current upon which an alternating current is superimposed. The parts that are to be coated are preferably treated at voltages up to 500 volts, more preferably up to 350 volts. The treatment may be a single treatment or it may be repeated. The anodic oxidation may proceed for instance by once or repeatedly immersing the parts in the electrolyte bath or by performing the treatment continuously to the extent the parts that are to be coated lend themselves to this variant of the method (for instance by continuously drawing foil or sheet material through the electrolyte bath or conveying objects through the bath on an aluminium suspension, etc.). Naturally the objects which are to be treated by the present method are electrically connected as the anode.

The tank containing the electrolyte will conveniently function as the cathode but there would naturally be no objection to the use of an insulated cathode. The cathode material does not detectably affect results, excepting when alternating current is used, in which case both electrodes should preferably be of aluminium, unless they are separated by a diaphragm.

The present method is preferably performed at a temperature of from C. to 95 C., more preferably from 15 C. to 40 C., so that the employment of heated or cooled baths can generally be avoided. The time needed for coating has generally been found to be from 0.1 to 30 minutes, preferably from 0.5 to 5 minutes, to give satisfactory results.

In the present method, the voltage may remain constant or it may gradually be controllably raised up to higher values as the treatment proceeds.

Constant voltage treatments are usually preferred when the process is continuous. If the voltage used is from 180 to 350 volts, then the resultant layer will have a thickness of from 8 to 50 microns, depending upon the duration of the treatment. On the other hand, if a lower voltage is applied, a layer thickness up to about 5 microns will be obtained, also varying with the duration of the treatment.

If the voltage is gradually raised during the anodising treatment only a batchwise process will be possible, the thickness of the resultant coating again depending upon the duration of the treatment and upon the level up to which the voltage is raised.

The current density employed affects the thickness, porosity, adhesive strength and uniformity of the resultant coatings. The coatings will be the thicker and the more porous the higher the current density and at the same time its power of adhesion and its uniformity will both lessen.

The distance between the electrodes is relevant only in the production of coatings of great porosity, uniformity and fineness, for which purposes the electrode spacing 4 should be close and the cathode/ anode ratio should preferably be 1 or more.

The aluminium oxide surfaces obtained by the present method have a number of outstanding properties and are largely stable to alkaline media and cold water, but not to attack by mineral acids and boiling water; nor are they attacked by organic solvents. No silicate could be detected in the coatings. Moreover, the coatings have electrical insulating properties and, according to layer thickness, their breakdown resistance may be asvhigh as 500 volts. In thicknesses upwards of a few microns, the coatings have a white appearance and they exhibit excellent adhesive bonding abilities. Moreover, they are porous and their powers of absorption are therefore high. The present method permits very thin films which are practically incapable of being measured as well as coatings up to 50 microns thick to be obtained.

The present method therefore enables aluminium and aluminium alloys to be used for a wide diversity of purposes. For example, coated aluminium plates provide useful offset printing plates, though a special alloy must be used, the hitherto conventional methods of producing such offset printing plates being multistage processes performed in acid baths. The present method permits a 6 to 8 micron coating which externally is of very much the same appearance to be produced by a method comprising not more than two stages, the first stage possibly being a cleaning operation.

In the production of offset printing plates, the best results are achieved by very slowly notching up the voltage at bath temperatures of preferably 45 C. to 50 C.

For coating an aluminium sheet or an aluminium foil that is particularly suitable for adhesive bonding, the gradual raising of the voltage, preferably up to from to volts has proved most satisfactory; With advantage the thickness of the coating may be up to about 2 microns.

An advantage of the present method is not only the considerably lower cost and longer life of the electrolyte bath compared with a chromate sulphuric acid bath, but also in the non-hazardous preparation of the bath and in the absence of difiiculties in connection with its disposal, no efiluent problem arising. It has also been found that at a current density of 0.5 amp./dm. to 3 amp./dm. good layers are formed with satisfactory speed. According to the size of the workpieces and the capacity of the current source, a working cycle of from 0.5 to 10 minutes, preferably from 0.5 to 5 minutes, can be achieved. It will therefore be understood that another advantage of the present method is the substantially higher production rate and hence capacity of the plant.

Aluminium parts that had been coated by the present method were submitted to conventional tests. It transpired that the quality of the adhesive bond was superior to that of a sheet oxidised by a chemical pickling method. It was also found that objects treated by the present method had a longer storage lifeat least 3 weeks-than objects that had been subjected to the pickling proce s and that must usually be further processed within 24 and not more than 48 hours.

The present method therefore permits sheet and foil material for adhesive bonding purposes to be supplied in roll form. Another advantage is that for producing by the present method coatings that can be adhesively bonded, a treatment lasting 1 to 3 minutes gives the best results, whereas in the pickling method the treatment time for optimum quality is about 30 minutes.

Owing to their excellent ability to be adhesively bonded,,

aluminium foils or sheets thus produced can be used in the making of skis, in the aircraft industry as well as in the construction of containers and, generally speaking, wherever sandwich-type and composite elementsof aluminium or aluminium alloys, possibly in association with plastics and paper, are needed.

" An electrolytic bath of the following composition:

240 g. of sodium waterglass (34%) 40 g. of ethylene glycol,

40 gQof sodiumtartrate and 3700 g. of water,

is placed in a plastics container of 4 litres capacity.

The cathode is a '5 x cm. steel plate and the anode a S x 10 cm. aluminium plate 0.3 mm. thick. The bath temperature is 25 C. and a dc. voltage of up to 150 volts is applied. The voltage is raised within a minute from 0 to 150 volts and anodising is continued at this voltage for one more minute. Owing to the development of an oxide film on the anode, the current fails. By gradually raising the voltage, the current density can be kept at about 3 amp./dm'. After completion of the oxide coating, the aluminium plate is taken out of the bath, washed first with tap water, then with distilled water and finally rinsed with acetone and dried. A transparent light film has'formed in which no optically visible pore structure is apparent. Using a template, silver varnish is sprayed on the coated aluminium plate so that a surface of 1 sq. cm. is obtained. The film of silver varnish is contacted with'a copper wire and the test condenser thus obtained measured. Its capacity is 2000 pf. and the loss factor about -30.10'

Example 2 The experimental set-up is the same as in Example 1, but the electrolyte has the following composition:

1 G. Phenol (98%) -2 40 Caustic soda solution 40 Sodium tartrate 40 Sodium waterglass (34%) 480 Water n 3400 The DC voltage is raised in one minute from 0 to 260 volts. In a bath of this composition, the breakdown voltage of the layer in the electrolyte bath is 280 volts. The coating obtained has a homogeneous appearance and its insulation capacity is 380 volts A.C.

Example 3 v The experimental set-up is again the same as in Example 1, but the composition of the electrolyte is as A coating is produced using a rising voltage and for different periods of time. The capacitiesof condensers thus produced are listed in Table 1 as a function of voltage, the treating time and the .temperature of the bath.

TABLE 1 Capacity Time, per sq Temperamins. cm., pf. ture, C

Example 4 The experimental set-up is again the same as that described in Example 1. The electrolyte has the following composition:

G. Sodium potassium tartrate 40 Sodium hydroxide solution 40 Sodium waterglass (34%) 180 Potassium waterglass (25%) 240 Water 3500 A hard aluminium foil (99.5% Al), 100 microns thick is coated. The voltage is raised in 2 minutes from 0 to 200 volts and the treatment continued at this voltage of 200 volts for another 8 minutes.

The resultant coating is washed and dried, covered with a PVC-based lacquer and dried for one minute at 130 C. The adhesion of the lacquer is excellent and it was found that, compared with untreated aluminium, the improvement in adhesive power is considerable. No loss of adhesion was detected after 240 hours immersion in water.

EXAMPLE 5 Again the same experimental set-up as in Example 1 is used, the electrolyte having the following composition:

G. Polyethylene glycol 400 200 Sodium hydroxide solution 40 Sodium potassium tartrate 120- Sodium waterglass 480 Water 3200 Aluminium strip, 10 cm. long, '2 cm. wide and 1 mm. thick is coated at a voltage of 100 volts for 2 minutes. The strip consists of an aluminium alloy in which the principal alloying elements are zinc, magnesium or silicon. Before being coated, the strip has been treated with a degreasing agent, washed with water and rinsed with acetone.

This preliminary treatment proves to give the best results. The acetone may be replaced by trichloroethylene or some other chlorinated hydrocarbon conventionally used for degreasing.

The coated aluminium strip is then bonded by a phenolic resin adhesive, the bonded surface being 2 sq. cm. Adhesive bonding is carried out at 140 C. under 10 kp./ per sq. cm. pressure for 8 minutes and the bonded strips are stored at room temperature for 24 hours. The following tensile test shows that the present method enables bonding strengths to be achieved which are better thanthe I best values obtained by methods at present conventionally It will be' understood from this Table that a treating used. For instance, the breaking strength of samples produced by the present method is 250 kg./sq. em. whereas the peak values obtained with samples (pickling process) produced by conventional methods do not exceed 200 kp./sq. cm.

EXAMPLE 6 An electrolytic bath is produced by diluting an ap proximately 34% solution. of sodium waterglass with tap water to 8% and adding 5% triethanolamine. This,

' electrolyte solution is placed in a plastics tank of 10 litres plate that is to be treated and the latter is immersed in the bath. The bath is kept in a state of considerable turbulence with the aid of an agitator. A D.C. voltage of a few volts is then applied and gradually raised to keep the current constant. At a current density of 3 ampjdm. a voltage of 180 to 200 volts is thus reached in 30 seconds. Since any further voltage increase leads to flashover, the voltage rise is stopped at between 180 and 200 volts. Upon this voltage being reached, the current is allowed to fall to a density of 0.1 amp./dm. and at this level the DC. source is switched off. The aluminium plate in then taken out of the bath, rinsed in water and dried which is normally effected in a current of air which may be hot or cold. The dry aluminium plates can at once be adhesively bonded, no after-treatment being required. For this purpose two aluminium plates are each completely covered on one side with an epoxy resin adhesive of type AW 136 made by Ciba and then bonded by the application of a pressure of kp./ sq. cm. at a temperature of 130 C., the pressure being maintained for 10 minutes. The temperature is then gradually allowed to drop to 40 C. whilst the pressure of 10 kp./sq. cm. continues to be maintained. The cooled pressings are then submitted to a bonding strength test, the force being measured which is required to separate the aluminium plates by peeling. The test results are assessed by reference to the peeling force and also by examination of the exposed surface. If the bond is satisfactory the adhesive must not have parted from the aluminium surface. In other words, the strength of the adhesive bond to the aluminium surface must be greater than the breaking strength of the adhesive material. In the case of the aluminium samples produced according to Example 1 the result of the peeling test was good, only about of the aluminium oxide layer having a bonding strength that was less than that of the adhesive.

EXAMPLE 7 An electrolyte solution similar to that in Example 6 but containing 7% triethanolamine is produced and, in an analogous manner, 10 x cm. aluminium plates of 0.5 mm. thickness are anodicaly oxidised in the bath. The oxidised plates are washed and dried and bonded by an epoxy resin adhesive of type AW 106 made by Ciba, a rubber band being interposed between the two aluminium plates. The peeling tests gave very good uniform values for breaking strength and there was no separation of the adhesive from the aluminium oxide surface. In this instance the rubber band was torn apart in such manner that a like rubber surface remained on both plates.

EXAMPLE 8 An electrolyte solution is produced by diluting a solution of waterglass to 8%, with an addition of 7% of monoethanolamine. In a 30 litre capacity tank made of steel sheet and also serving as the cathode, six 10 x 30 cm. aluminium plates are anodised using a current density of 1.5 amp./dm. A voltage of 180 volts is reached in 2 minutes and, after the current has been switched oif, the plates are taken out of the tank, washed with water and dried. Bonding is effected with an epoxy resin adhesive of the type AW 136 made by Ciba, a rubber film being interposed and bonded between the two aluminium plates. The following test gave particularly good results. After peeling, both plates exhibited an unexceptionable rubber surface. The breaking strength was characterised by a. curve of very uniform shape.

EXAMPLE 9 A plastics tank 150 mm. wide, 1100 mm. high and 1100 mm. long, is filled with an electrolyte of the following composition:

45 kg. of sodium waterglass (34% 10.5 kg. of monoethanolamine,

1.5 kg. of sodium potassium tartrate, and 110 litres of water.

8 The cathode is an x cm. steel sheet 1 mm. thick, and an aluminium sheet that is to be coated is 0.3 mm.

thick, 975 mm. long and 755 mm. wide. An anode is contacted with the aluminium sheet with the aid of three suction electrodes having a diameter of 10' cm., and then immersed into the tank, coating being performed in 30 minutes by using a pulsed current supplied by a thyristorcontrolled rectifier. The voltage is raised from 0 to 220' Since the best results are achieved at a bathtemperature of 45 C. to 50 C. it is necessary to heat up the bath at 45 C. before starting thetreatment and this is accomplished by using large electric immersion heaters. In order to keep the bath at this temperature during the process of anodisation, the bath is cooled by circulating the electrolyte through a heat exchanger during the anodising process. This enables the temperature of the bath to be kept constant between 45 C. and 50 C. during anodisation. The treatment is completed at the end of 30 minutes, as already stated. The sheet is lifted out of the bath, the suction electrodes are disconnected and finally the coating is washed with water. In order-to dry the sheet quickly, the layer is rinsed with acetone and then dried in air. The following printing tests showed that the layer that had been produced by the method according to the invention was wellsuited for use as an offset printing plate. The printing tests were carried out by four-colour printing and it was found that the layer gave excellent resolution, high printing stability and carried water very well so that the work could be,

very satisfactorily done. Another advantage of a layer produced according to the invention is its substantially lighter whitish grey colour which provides a better contrast for copying.

is placed in a 20 x 20 cm. plastics tank which is cm. high. The cathode is a steelsheetlOO cm. long, 15 cm. broad and 1 mm. thick. An aluminium plate that is to be coated consists of a 99.5% aluminium and is 100 cm. long, 10 cm. wide and 1 mm. thick. The coating treatment is performed by slowly immersing the aluminium plate into the bath with the application of a voltage of 200 volts. With a violent display of sparks and flashovers between the metal surface and the electrolyte a white film immediately forms and the resultant insulating effect of this layer soon causes the sparking and flashovers between the electrode surface and the electrolyte to be reduced to a level at which only very small sparks can be seen. The rate of immersion is usually high enough to achieve a current density of 3 amp./dm. to 5 amp./dm. The rate of immersion thus primarily depends upon the performance of the rectifier and the thickness of the.

Finally the coated electrode is washed with water, rinsed with acetone and dried in air. A test of the breakdown resistance of this layer proves that it has an insulating effect up to 500 volts A.C.

EXAMPLE 11 A 34% solution of waterglass is diluted to 5% with demineralised water, and 4% monoethanolamine and 1% sodium fluoride are added as a complex-forming substance, the volume of the solution being litres. Aluminium foil of 0.2 mm. thickness and 50 mm. width is coated. A 20 cm. length of foil is slowly dipped into the solution at avoltage of 220 volts. Immersion proceeds at a rate at which the current will not exceed 5 amp. The entire foil is immersed in the bath in 15 seconds. The foil is then taken out and the coating process is repeated at a voltage of 250 volts. Again the rate of immersion is so chosen that the current is about 5 amp./dm. When the second coating treatment has been completed, the foil is taken out of the bath, washed with water and then dried. The foil is now covered with a uniform white film which has an excellent bonding strength, and which is very suitable for decorative purposes, for instance as a wallcovering.

EXAMPLE 12 In the same manner as described in the preceding Example, an electrolyte solution containing 1% sodium waterglass, 5% triethanolamine, 1% methyl pyridine and 20% acetonitrile is prepared with demineralised water. An aluminium foil 0.1 mm. thick, 50 mm. wide and 20 cm. long is slowly immersed in this solution with the application of a voltage of 180 volts, the rate of immersion being so chosen that the current does not exceed 3 amp./drn. In half a minute the immersion is completed. This process is repeated at 220 volts and at 250 volts, the current being again held at 3 amp. After the third immersion, the foil is washed with demineralised water and dried. The layer thickness is 35 microns and the breakdown resistance is about 400 volts A.C. Strips 20 X 50 mm. are cut from this foil and two such strips are superimposed with the interposition of a paper fleece that has been impregnated with a boric acid electrolyte. The capacity is then determined with the aid of an RC-bridge circuit. The measurements gave 1500 pf./sq. cm., a relatively high value for a product of such a kind.

What is claimed is:

1. A method of coating a part or object made entirely or partly of aluminum or an aluminum alloy comprising submitting the part to be coated to anodic oxidation in an electrolyte bath containing in solution at least one bath-soluble alkali metal silicate, a liquid electrolyte selected from the group consisting of water and mixtures of water and at least one organic solvent and at least one bath-soluble organic complex-forming substance selected from the group consisting of water-soluble primary, secondary and tertiary amines, water-soluble salts of amino acids, water-soluble salts of carboxylic acids, watersoluble salts of sulfonic acids, water-soluble monoand polyhydric phenols, and water-soluble polyhydric alcohols, with the application of a direct current, an alternating current or a pulsating current.

2. A method as claimed in claim 1, in which the organic complex-forming substance is a primary, secondary or tertiary amine.

3. A method as claimed in claim 4, in which the amine is an alkanolamine.

4. A method as claimed in claim 3, in which the alkanolamine is mono-, dior triethanolamine.

5. A method as claimed in claim 1, in which the organic complex-forming substance is from the group consisting of water soluble salts of an organic carboxylic acid which may contain multiple bonds, a salt of a substituted carboxylic acid, an amino acid, a substituted amino acid, a sulphonic acid, a substituted sulphonic acid, a monoor polyhydric substituted or unsubstituted phenol, a water soluble, polyhydric substituted or unsubstituted alcohol, and a mixture thereof.

6. A method as claimed in claim 1, in which the organic complex-forming substance is from the group 10- consisting of 2-amino-propanol, B-dimethyI-Z-aminoethanol, a salt of ethylenediamino-tetraacetic acid, a salt of cyclohexanediamino-l, Z-tetraacetic acid, a salt of nitrilotriacetic acid, a pyridine-2,6-dicarboxylic acid, 2- pyridyl hydrazine, a pyridine-3 sulphonic acid, pyrrolidone, a pyrrole-2-carboxylic acid and pyrimidine.

7. A method as claimed in claim 1, in which the organic complex-forming substance is from the group consisting of a water soluble salt of glycine, alanine, glutamic acid, tryptophan, methionine, tyrosine, 3-bromotyrosine, aspartic acid, oxylysine and oxyproline.

8. A method as claimed in claim 1, in which the organic complex-forming substance is from the group consisting of a water soluble salt of maleic acid, furnaric acid, acrylic acid, methacrylic acid, p yromellitic acid, citric acid, tartaric acid, aconitic acid and cinammic acid.

9. A method as claimed in claim. 1, in which the organic complex-forming substance is from the group consisting of a water soluble salt of benzenesulphonic acid, toluenesulphonic acid and lignosulphonic acid.

10. A method as claimed in claim 1, in which the complex-forming substance is from the group consisting of phenol, cresol, resorcinol, 2,4,6-trinitroresorcinol, phloroglucinol and pyrogallol.

11. A method as claimed in claim 1, in which the complex-forming substance is from the group consisting of ethylene glycol, propylene glycol, polypropylene glycol, glycerol and nitropropane-diol.

12. A method as claimed in claim 1, in which the anodic oxidation is carried out with direct current.

13. A method as claimed in claim 1, in which the anodic oxidation is carried out with a pulsating current.

14. A method as claimed in claim 1, in which the anodic oxidation is carried out with a direct current and with a superimposed alternating current.

15. A method as claimed in claim 1, in which the anodic oxidation is carried out with alternating current.

16. A method as claimed in claim 1, in which the concentration of bath soluble silicate in the electrolyte bath is from 0.1 to 15% of the bath.

17. A method as claimed in claim 1, wherein the concentration of bath soluble silicate in the electrolyte bath is from 5 to 9%.

18. A method as claimed in claim 1, wherein the concentration of complex-forming substance in the bath is from 0.1 to 40%.

19. A method as claimed in claim 1, wherein the concentration of complex-forming substance in the bath is from 1 to 12%.

20. A method as claimed in claim 1, wherein the temperature of the bath is from 0 C. to C.

21. A method as claimed in claim 1, wherein the temperature of the bath is from 40 C. to 70 C.

22. A method as claimed in claim 1, wherein anodic oxidation is carried out in an electrolyte bath containing 7 to 10% sodium silicate and 7 to 9% monoethanolamine.

23. A method as claimed in claim 1, wherein the aluminum alloy submitted to anodic oxidation contains at least 5% aluminum.

24. A method as claimed in claim 1, wherein plastics film or paper having one surface from the group consisting of aluminum and an aluminum alloy is submitted to anodic oxidation.

25. A method as claimed in claim 1, wherein the anodic oxidation is performed at constant voltage.

26. A method as claimed in claim 1, wherein during the anodic oxidation the voltage is raised to fiashover point.

27. A method as claimed in claim 1, wherein the process of anodic oxidation is repeated.

28. A method as claimed in claim 1, wherein the current density is kept at from 0.5 to 3 amp./dm.

29. A method as claimed in claim 1, wherein the 11 duration of the anodic oxidation process is from 0.1 to 30 minutes.

30. A method as claimed in claim 1, wherein the duration of the anodic oxidation process is from 0.5 to 5 minutes.

References Cited UNITED STATES PATENTS 3,658,662 4/1972 Casson et a1. 20458 1 2 2,364,964 12/ 1944 Frasch 2045 8 3,445,353 5/ 1964 Harendza-Harinxma 2045 8 FOREIGN PATENTS 727,749 4/1955 Great Britain 20458 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner UNI'IED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,824,159 I I bated 'July 16, 197 4- Inventor(s) Felix nn et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Inv the heading to the printed specification, line 9, "'A 4,295/71" should read 6A 4295/71-1-2 Column 9,

line 60, c1'aim 3, "in claim 4 should read in claim '2 Signed andi'sealed this 19th day of November 1974.

(SEAL) tteStr,

McCOY M; GIBSON JR. I .c. MARSHALL DANN; i Attesting ()fficier Commissioner of Patents 

